linux-kernel - [tip:timers/core] seqcount: Introduce raw_write_seqcount

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Date:	Thu, 18 Jun 2015 15:19:27 -0700
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, tglx@...utronix.de,
	viro@...IV.linux.org.uk, linux-kernel@...r.kernel.org,
	hpa@...or.com, oleg@...hat.com, peterz@...radead.org,
	mingo@...nel.org, paulmck@...ux.vnet.ibm.com
Subject: [tip:timers/core] seqcount: Introduce raw_write_seqcount_barrier(
 )

Commit-ID:  c4bfa3f5f906aee2e084c5b1fb15caf876338ef8
Gitweb:     http://git.kernel.org/tip/c4bfa3f5f906aee2e084c5b1fb15caf876338ef8
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Wed, 17 Jun 2015 14:29:24 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 19 Jun 2015 00:09:56 +0200

seqcount: Introduce raw_write_seqcount_barrier()

Introduce raw_write_seqcount_barrier(), a new construct that can be
used to provide write barrier semantics in seqcount read loops instead
of the usual consistency guarantee.

raw_write_seqcount_barier() is equivalent to:

	raw_write_seqcount_begin();
	raw_write_seqcount_end();

But avoids issueing two back-to-back smp_wmb() instructions.

This construct works because the read side will 'stall' when observing
odd values. This means that -- referring to the example in the comment
below -- even though there is no (matching) read barrier between the
loads of X and Y, we cannot observe !x && !y, because:

 - if we observe Y == false we must observe the first sequence
   increment, which makes us loop, until

 - we observe !(seq & 1) -- the second sequence increment -- at which
   time we must also observe T == true.

Suggested-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: umgwanakikbuti@...il.com
Cc: ktkhai@...allels.com
Cc: rostedt@...dmis.org
Cc: juri.lelli@...il.com
Cc: pang.xunlei@...aro.org
Cc: oleg@...hat.com
Cc: wanpeng.li@...ux.intel.com
Cc: Al Viro <viro@...IV.linux.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20150617122924.GP3644@twins.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 include/linux/seqlock.h | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index c07e3a5..486e685 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -233,6 +233,47 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
 	s->sequence++;
 }
 
+/**
+ * raw_write_seqcount_barrier - do a seq write barrier
+ * @s: pointer to seqcount_t
+ *
+ * This can be used to provide an ordering guarantee instead of the
+ * usual consistency guarantee. It is one wmb cheaper, because we can
+ * collapse the two back-to-back wmb()s.
+ *
+ *      seqcount_t seq;
+ *      bool X = true, Y = false;
+ *
+ *      void read(void)
+ *      {
+ *              bool x, y;
+ *
+ *              do {
+ *                      int s = read_seqcount_begin(&seq);
+ *
+ *                      x = X; y = Y;
+ *
+ *              } while (read_seqcount_retry(&seq, s));
+ *
+ *              BUG_ON(!x && !y);
+ *      }
+ *
+ *      void write(void)
+ *      {
+ *              Y = true;
+ *
+ *              raw_write_seqcount_barrier(seq);
+ *
+ *              X = false;
+ *      }
+ */
+static inline void raw_write_seqcount_barrier(seqcount_t *s)
+{
+	s->sequence++;
+	smp_wmb();
+	s->sequence++;
+}
+
 /*
  * raw_write_seqcount_latch - redirect readers to even/odd copy
  * @s: pointer to seqcount_t
--
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